This application claims priority from Japanese Patent Application 2002-273009, filed Sep. 19, 2002, which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a control for a vehicle having an automatic transmission.
2. Description of the Related Art
In an automatic transmission of a vehicle, a speed change mechanism is controlled by means of oil pressure from an oil pump driven by an engine. For example, in a V-belt continuously variable transmission (CVT), a V-belt is gripped by means of the oil pressure supplied to a primary pulley and a secondary pulley, to transmit a torque.
In a vehicle mounted with the V-belt CVT, when the vehicle is stopped for a long time at a cold place, it is not rare that an oil temperature of the transmission drops below xe2x88x9220xc2x0 C. When the engine is started and the vehicle runs in this condition, the oil pressure is supplied to the primary pulley and the secondary pulley and speed change is performed as the vehicle speed rises. At this time, since a viscosity level of the operating oil is great, the supply pressure does not increase, and the force of the pulleys gripping the V-belt is insufficient, thus creating a possibility of slippage of the V-belt.
According to JP-2735129-B, published by the Japan Patent Office in 1998, when the oil temperature in the transmission decreases below a predetermined extremely cold temperature, a torque reduction is requested to the engine, and the engine output is restricted and the torque inputted into the transmission is reduced, to prevent the belt slippage.
However, the above-mentioned conventional example presumes the case where the transmission is operated with the oil pressure in the state of the lowest transmission oil temperature, and the torque reduction amount requested to the engine is a pre-set fixed value. Therefore, as the oil temperature rises, the engine torque reduction becomes excessive.
It is therefore an object of this invention to set the torque reduction amount to an appropriate amount, in the case where the engine torque is reduced when the oil temperature is the extremely cold.
According to the present invention, there is provided a control system for an automatic transmission, which includes an oil pressure control mechanism for supplying an oil pressure to the transmission, and a sensor for detecting the oil pressure supplied from the oil pressure control mechanism to the transmission. A controller of the control system calculates a maximum torque transmittable by the transmission based on a speed ratio of the transmission and the detected oil pressure, calculates an engine torque reduction amount from the difference between the torque being inputted into the transmission and the maximum transmittable torque, and reduces the engine torque according to the engine torque reduction amount.
Embodiments of the present invention, and merits of the present invention, are explained in detail below, with reference made to the attached drawings.